The present invention relates to metal coated abrasive particles or grit and more particularly to such coated grit for improving its retention in saw blade segments.
Superabrasive grit, such as diamond particles, is widely used in sawing, drilling, dressing, grinding, lapping, polishing, and other abrading applications. For these applications, the grit typically is surrounded in a matrix of a metal, such as Ni, Cu, Fe, Co, Sn, W, Ti, or an alloy thereof, or of a resin, such as phenol formaldehyde or other thermosetting polymeric material. By attaching the matrices to a body or other support, tools may be fabricated having the capability to cut through such hard, abrasive materials as concrete, asphalt, masonry, ceramic, brick, or granite, marble, or other rock. Exemplary of such tools is a saw blade as disclosed in U.S. Pat. No. 4,883,500 to be comprised of a circular steel disk having a number of spaced segments disposed about its cutting edge. Each of the segments includes an amount of diamond grit dispersed in a suitable bonding matrix of a metal or alloy, such as Co, bronze, cemented metal carbide, or the like.
The retention of diamond grit within the matrix primarily involves a mechanical component developed from the surrounding of the individual particles by the matrix. The exposure of the grit, accordingly, must be limited so as not to critically weaken the mechanical properties of the surrounding matrix on the grit particles. For example, in a typical saw blade application, the average exposure of the grit is less than about 20% of the total grit height. The limited grit exposure, in turn, limits the cutting rate. Moreover, as the matrix is ablatively worn during the use, the service life of the tool may be shortened as up to two-thirds of the grit is prematurely lost by its being pulled or popped out of the surrounding matrix.
In an attempt to improve grit retention, it has been proposed to coat diamond particles with carbide-forming transition metals, such as Mo, Ti, and Cr. Such metals typically are chemically vapor deposited (CVD) or sputtered onto the surfaces of the diamond grit. Examples of such coatings and processes for the deposition thereof are disclosed in U.S. Pat. Nos. 3,465,916; 3,650,714; 3,879,901; 4,063,907; 4,378,975; 4,399,167; 4,738,689; in U.S. Reissue No. 34,133; and in EP-A79/300,337.7. It has been reported, however, that these coatings may be oxidized and, depending upon the carbide formed, may be brittle. In response, proposals have been made to use a carbide-forming metal layer as part of a multi-layer coating system. As is described in U.S. Pat. Nos. 3,929,432; 5,024,680; 5,062,865; and 5,232,469, systems generally involve the vapor-phase deposition of an inner layer of a thin, 0.05 to 15 micron thick carbide-forming metal, and an outer layer of a more corrosion resistant metal, such as Ni or Cu for protecting the inner layer from oxidation.